This invention relates to split cycle internal combustion engines as described in my Australian Pat. Nos. 286539 and 416564 and my corresponding U.S. Pat. No. 3,408,811, British Patent Nos. 1,120,248 and 1,190,948, Canadian Patent No. 804343, Japanese Utility Model No. 50-22161, Italian Patent No. 725787, French Patent No. 1,443,008, and West German Patent Application No. 1,576,035.
The major characteristic of this kind of engine is the splitting of the cycle of operations into two separate groups, performed respectively in cylinders kept hot and in cylinders kept cold. All engines working on this split cycle compress the charge air to a very high pressure in a compressor consisting of two or more cold cylinders arranged in series of progressively decreasing size operating at progressively increasing pressure, the air being cooled as it passes from each cylinder to the next higher cylinder in pressure. The highly compressed air from the final compression stage is then passed to at least one small firing cylinder kept hot, in which the fuel is ignited to provide the power for the engine. The firing cylinder has minimal working clearance between the top of the power piston and the cylinder head, and the combustion dome is of small capacity. The high pressure air with fuel entrained begins to enter through a small transfer valve when the power piston is at or near top dead centre, and continues to enter and burn while the piston is moving down the first part of its power stroke. Engines of this construction are herein referred to as "split cycle engines of the kind described".
Each firing cylinder may have alongside it a larger cylinder to which it is connected by an insulated port low down its bore, which port is uncovered by the power piston as it continues its downward stroke. This is known as compound expansion, and when in use a major portion of the combustion charge, after being partly expanded in the firing cylinder, is passed through the port to the expansion cylinder for further expansion. A poppet valve in the head of each cylinder then exhausts the spent combustion products to the atmosphere.
Engines working on the split cycle principle normally operate at high compression pressures up to four or five times the compression pressures in conventional spark plug ignition petrol engines. Experience has shown that, while spark plug ignition engines of the split cycle type show the expected high efficiency and freedom from pollution, the spark plugs must operate at higher than conventional voltage, and so have a much shorter life span than in conventional engines. With conventional hot plug or incandescent tube ignition, the firing of the engine is somewhat erratic, and it is not possible to vary the ignition timing in response to variations in the load on the engine.
Accordingly, it is the principal object of the present invention to provide split cycle engines of the kind described having ignition means which can be varied with precision to accommodate variations in the load upon the engine, and which only require maintenance at approximately the same intervals as the remaining components of the engine.
It is a further object of the present invention to exploit to a greater degree than in the already proposed split cycle engines the potential of this type of engine for perfection in balance and reduction in bearing and piston friction.